"It suggests we've got old stuff," Capo said in a statement.
While the existence of manure alone does not prove that the animals housed in the corral were horses, there is strong evidence that they were, says team member Sandra Olsen of the Carnegie Museum of Natural History in Pittsburgh.
Her team recovered more than 300,000 animal bone fragments in just one of the five Kazakh villages being studied.
"Of those, over 99 percent are from horses," she said.
Nor are there any signs that the villagers hunted horses rather than raising them.
The researchers found horse skulls and backbones in the villages, indicating that horses were butchered on site.
(Related news: "Ice Age Horses May Have Been Killed Off by Humans, Study Finds" [May 1, 2006].)
If the horses had been hunted, Olsen said, the hunters wouldn't have bothered to bring back such heavy, useless bones.
"We call it the 'schlep effect,'" she said.
She also notes that thong smoothers are among the most common tools found in the five villages.
Thong smoothers were likely used to make rawhide thongs, or strips, that could have been part of equestrian accessories such as bridles.
The most common tools were hide scrapers, "which you also use for leather thongs," she said.
The Kazakh village is one of two sites competing for the honor of being the first place where humans are thought to have domesticated horses.
The other is in the Ukraine, where large numbers of horse bones have been found along with worked pieces of antler that might have been cheek pieces for bridles.
In general, definitive evidence for the earliest dates of horse domestication has been difficult to pin down.
Many bridle parts have been discovered dating back to 2000 B.C.—1,600 years after the settlement of the Kazakh village—as well as chariotlike carts buried with horses.
"But [such carts are] an advanced thing," Olsen said. Even the saddle is a relatively new invention.
"If you look at classical Greek statues of people on horseback, they don't have saddles, horseshoes, or stirrups," Olsen said. "And that's in Greek times, in 400 B.C."
Also, the earliest horse domesticators probably made what little stable gear they used out of leather, which rapidly decomposes.
Therefore all the earliest evidence of horse domestication is indirect and should be taken with a grain of salt, says Marsha Levine of the McDonald Institute for Archaeological Research at the University of Cambridge in England.
In a recent 54-page article, Levine says that indirect evidence can be useful. But, she cautions, nothing must be assumed and everything must be tested.
She says that she looks forward to seeing the new results once they are published.
"I am not a chemist," she wrote in an email to National Geographic News, "so I can't judge the results although it appears to me that there could be a variety of explanations" for the elevated nutrient levels.
But soil in the ancient animal pen may contain additional secrets.
Only a week ago, the Carnegie Museum's Olsen says, she learned that the supposed corral's soil is ten times saltier than the surrounding earth. The probable source: horse urine.
Geochemists are also analyzing the soil for traces of fatty chemicals unique to horse manure.
"If we find those," Olsen said, "that really nails it."
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